"Gradient PLUS surface temperature

What part of simple school-grade maths don't you understand?

Y=mX +C"

The gradient isn't a constant and depends upon numerous factors:
"However, the rate of increase with depth (geothermal gradient) varies considerably with both tectonic setting and the thermal properties of the rock."
http://www.enotes.com/earth-science/geothermal-gradient

So, we going to leave out "tectonic setting" or "hotspots" due to lack seeing this on Mars as of yet, but that still leaves "thermal properties of the rock" as a variable.
And this seems to vary a lot.
Example:

"The tectonically stable shield areas and sedimentary basins have average gradients that typically vary from 0.82–1.65°F/100 ft (15–30°C/km)."
http://www.enotes.com/earth-science/geothermal-gradient

In Utah:
http://geology.utah.gov/emp/geothermal/pdf/utah_temp_grad.pdf
On page 4 it depicts borehole temperatures in numerous locations and these are marked with a key indicating different temperature gradient for various locations.

The blue dot is for G < 25°C/km and red dot is G > 500°C/km [hottest hotspots, I suppose]
And green: 25 < G < 45°C/km
And white: 45 < G < 65°C/km
etc.

It seems likely to me that Mars should have a different gradient than Earth. But like Earth, Mars should have areas with different gradients which would depend upon numerous factors, but one them being the "thermal properties of the rock".

And that if one is dealing with depths starting of somewhere around 1 km down, then is seems to me the "thermal properties of the rocks" is going to be a more significant factor than the surface temperature.

And finally, no one measures the surface temperature on Earth, but instead they could use the average temperature in relation to any geothermal gradient.
So to do this "simple high school math", I need the average temperature at the poles on Mars.